1. Field of the Invention
The present invention relates to a gas bearing spindle, more particularly, a gas bearing spindle used in precision machining equipment, a hole drilling machine, an electrostatic coating machine, or the like.
2. Description of the Background Art
In a gas bearing spindle including a gas bearing for supporting a rotation shaft relative to a housing by supplying a compressed gas such as compressed air to a small clearance between the rotation shaft and a member opposite to the rotation shaft within the housing, the rotation shaft is supported relative to the housing in a non-contact manner. Hence, not only friction loss is small in the bearing but also the member is not fatigued or worn out in a normal operation state because the rotation shaft and the member opposite to the rotation shaft are not in direct contact with each other. Taking advantage of such a feature, the gas bearing spindle is widely used as a high-speed spindle used in precision machining equipment, a hole drilling machine, an electrostatic coating machine, or the like.
There have been made various proposals for improved performance of such a gas bearing spindle. For example, a proposed gas bearing spindle employs a sleeve having an inner wall formed of graphite as the member opposite to the rotation shaft, in order to avoid seizing of the rotation shaft even if the rotation shaft and the member opposite to the rotation shaft are brought into contact with each other. Meanwhile, a gas bearing spindle may be adopted which is capable of absorbing whirling vibration of the rotation shaft by supporting a sleeve, which serves as the member opposite to the rotation shaft, relative to the housing by means of an O ring (for example, see Japanese Patent Laying-Open No. 2002-295470 (Patent Document 1) and Japanese Patent Laying-Open No. 2007-170534 (Patent Document 2)).
FIG. 5 is a schematic cross sectional view showing an exemplary conventional gas bearing spindle. Referring to FIG. 5, the exemplary conventional gas bearing spindle will be described.
Referring to FIG. 5, conventional gas bearing spindle 100 includes a rotation shaft 110; a sleeve 130 having a sleeve through hole 133, which is a cylindrical through hole that surrounds a portion of an outer circumferential surface 111A of rotation shaft 110; and a housing 120 surrounding sleeve 130 to retain sleeve 130 by means of O rings 141, 142 each formed of a rubber. Rotation shaft 110 and sleeve 130 are disposed with a small journal bearing clearance 113 therebetween.
Rotation shaft 110 has a shaft portion 111 cylindrical in shape; and a flange portion 112 formed in one end of shaft portion 111 and having a large disk-like shape with a diameter larger than that of shaft portion 111. In the other end of shaft portion 111, a retaining unit 119 is formed to retain a tool or the like. Sleeve 130 is provided with a plurality of journal nozzles 151 formed in the circumferential wall of sleeve 130 to supply a gas for bearing to a journal bearing clearance 113 provided between the inner circumferential surface of sleeve through hole 133 and outer circumferential surface 111A of shaft portion 111 of rotation shaft 110.
Journal nozzles 151 are arranged in two rows extending in the circumferential direction of sleeve through hole 133. Specifically, journal nozzles 151 are provided in the rows at respective sides that interpose therebetween the central portion of sleeve 130 in a direction in which sleeve through hole 133 extends (axial direction of shaft portion 111 of rotation shaft 110).
With the above-described configuration, sleeve 130 serves as a gas journal bearing for supporting rotation shaft 110 relative to housing 120 in a non-contact manner in a direction (radial direction) perpendicular to the axial direction of shaft portion 111. Furthermore, in housing 120, thrust bearings 160 annular in shape are disposed so that one base surface of each thrust bearing 160 is opposite to each of base surfaces 112A of the opposite sides of flange portion 112 of rotation shaft 110. Here, thrust bearing 160 and flange portion 112 of rotation shaft 110 are separated with a small thrust bearing clearance 114 therebetween. Thrust bearings 160 are provided with a plurality of thrust nozzles 161 for supplying a gas to thrust bearing clearance 114 provided between the one base surface of each thrust bearing 160 and each of base surfaces 112A of flange portion 112 opposite thereto. The plurality of thrust nozzles 161 are formed in a direction along the circumferential direction of flange portion 112.
Each of journal nozzles 151 is connected to a bearing gas supply path 121, which is formed within housing 120, via a sleeve gas supply path 152 and an annular space 122, which is a space closed by sleeve 130, housing 120, and O ring 142. On the other hand, each of thrust nozzles 161 is connected to bearing gas supply path 121 via a thrust bearing gas supply path 162. Further, bearing gas supply path 121 is connected to a bearing gas supply source, such as an air compressor, having a function of supplying a high-pressure gas such as air, disposed external to gas bearing spindle 100, and not shown in the figure.
Further, in a portion of flange portion 112, i.e., in an outer circumferential portion thereof, there is formed a thin portion 112B having an axial thickness thinner than that of adjacent portion in flange portion 112. On one base surface of thin portion 112B, turbine blades 115 are formed. Turbine blades 115 thus formed are arranged in the circumferential direction of flange portion 112, have a plate-like shape, and are adapted to receive an incoming gas to rotate rotation shaft 110 in the circumferential direction of flange portion 112. Furthermore, in housing 120, a turbine nozzle 173 is formed at an outer circumferential side relative to flange portion 112. Turbine nozzle 173 has an opening at its portion facing turbine blades 115, and is configured to be capable of jetting a drive gas such as a compressed gas from the inner wall of housing 120 toward turbine blade 115. Turbine nozzle 173 is connected to a drive gas supply path 171 via a circumferential groove 172 formed to extend in the direction along the outer circumference of flange portion 112. Drive gas supply path 171 is connected to a drive gas supply source, such as an air compressor, having a function of supplying a high-pressure gas such as air, disposed external to gas bearing spindle 100, and not shown in the figure. Furthermore, housing 120 is provided with a drive gas discharge path 175 having one opening and the other opening. The one opening is provided in the surface thereof at the side where turbine blades 115 on thin portions 112B of flange portion 112 are formed, specifically, is provided at a location opposite to a region at an inner circumferential side relative to the region in which turbine blades 115 are formed. The other opening is formed at an outer wall of housing 120.
Here, sleeve 130 includes a bearing portion 131 formed of a nonmetallic sintered body and retaining rings 132 formed of a metal. Bearing portion 131 has sleeve through hole 133, and has an outer circumferential surface, a portion of which constitutes the outer circumferential surface of sleeve 130. Bearing portion 131 is configured so that the inner circumferential surface of sleeve through hole 133 is opposite to outer circumferential surface 111A of rotation shaft 110. Retaining rings 132 are fit into the outer portions of the ends of the opposite sides of bearing portion 131 in the direction in which sleeve through hole 133 extends (axial direction of rotation shaft 110), so as to retain sleeve 130 relative to housing 120 by means of O rings 141, 142 each serving as an elastic member and formed of a rubber.
As such, sleeve 130 has bearing portion 131 formed of a sintered material such as graphite, so seizing of rotation shaft 110 can be prevented even if rotation shaft 110 and sleeve 130 are brought into contact with each other. Furthermore, since sleeve 130 is supported relative to housing 120 by means of O rings 141, 142, whirling vibration of rotation shaft 110 can be attenuated.
However, the above-described conventional gas bearing spindle has a problem as described hereinafter. FIGS. 6 and 7 are schematic cross sectional views showing one exemplary supporting structure of the sleeve in the conventional gas bearing spindle.
As described above, in the conventional gas bearing spindle shown in FIG. 5, in order to rotatably support rotation shaft 110 relative to sleeve 130 in a non-contact manner, a compressed gas is supplied to annular space 122 from a bearing gas supply unit not shown in the figure. Here, as shown in FIG. 6, a small clearance 190 is formed between each retaining ring 132 and bearing portion 131 in the axial direction. Clearance 190 communicates with annular space 122. Hence, when annular space 122 is supplied with the compressed gas, due to a difference from pressure of atmosphere, force is generated in the axial direction of retaining ring 132 in proportion to the area of a ring geometry having an outer diameter φD and inner diameter φd1. The force thus generated may cause deviation of retaining ring 132 relative to bearing portion 131 in a fit surface 180 therebetween. The deviation of retaining ring 132 causes deviation of the interval between retaining ring 132 and housing 120 from its original value, which may hinder elastic deformation of O rings 141, 142 upon whirling vibration of rotation shaft 110, thereby decreasing performance of gas bearing spindle 100.
To address this, the force generated in retaining ring 132 can be reduced by reducing the thickness of retaining ring 132 to provide retaining ring 132 with a larger inner diameter φd1. However, retaining ring 132 needs to be provided with grooves 132A for retaining O rings 141, 142, so it is difficult to make retaining ring 132 thinner significantly. Further, upon drilling sleeve through hole 133 in the process of manufacturing sleeve 130, it is preferable to retain sleeve 130 so that stress is imposed on retaining ring 132 formed of a metal and having a toughness higher than that of bearing portion 131 formed of a nonmetallic sintered body, but retaining ring 132 reduced in its thickness has a decreased rigidity and therefore sleeve 130 may be deformed by the stress when it retains sleeve 130.
Further, as shown in FIG. 7, if sleeve gas supply path 152 is formed to penetrate bearing portion 131 of sleeve 130 and fit surface 180 of retaining ring 132, bearing portion 131 and retaining ring 132 are deviated from each other at fit surface 180 in the rotation direction when rotation shaft 110 rotating fast are brought into contact with bearing portion 131. Accordingly, the communication between each of sleeve gas supply paths 152 provided in retaining ring 132 and each of journal nozzles 151 provided in bearing portion 131 are disconnected, thereby blocking sleeve gas supply paths 152. When each of sleeve gas supply paths 152 is thus blocked, the compressed gas cannot be supplied to bearing clearance 113, which disadvantageously makes it difficult to support rotation shaft 110 in a non-contact manner.